Nanocellulose aerogels and foams

ABSTRACT

The present disclosure provides a method for preparing an aerogel or a foam, the method comprising: forming a reaction mixture comprising a cellulose nanofibril gel, a first solvent, and one or more crosslinking agents under conditions sufficient to crosslink the gel; and contacting the crosslinked gel with a second solvent under conditions sufficient to dry the crosslinked gel, thereby forming an aerogel or foam.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.62/649,915 filed Mar. 29, 2018, which is incorporated herein in itsentirety for all purposes.

BACKGROUND OF THE INVENTION

Aerogels are uniquely ultra-light and ultra-porous materials with over99.5% porosity or volume filled with air debuted in early 1930s. Whilemost aerogels are silica or carbon based, those derived from sustainableand renewable materials are particular enticing, among which, celluloseaerogels present novel attributes such as low thermal conductivity,flexibility, excellent wet resiliency, etc. Cellulose aerogels have beenmost commonly produced by multi-step sol-gel processes involvingcellulose dissolution, regeneration in non-solvents and solvent exchangeto suitable media for either supercritical-drying or freeze-drying,demanding the use of large quantities of chemicals, such asN-methyl-morpholine-N-oxide (NMMO), alkali hydroxide/urea, sodiumhydroxide, calcium thiocyanate tetrahydrate, lithiumchloride/dimethylacetamine (LiCl/DMAc),¹¹ lithium chloride/dimethylsulfoxide (LiCl/DMSO) and ionic liquid, etc. As dissolution destroys thenative cellulose I crystalline structure, the regenerated structures areeither amorphous or cellulose II structure. Nanocelluloses are thecrystalline domains of original sources isolated without dissolution,thus retain the cellulose I crystalline structure and represent muchgreener and stronger precursor for aerogels. The as fabricatednanocellulose aerogels have been extensively investigated for air/waterpollutant removal, drug delivery, biomedical scaffolding,electrochemical applications, energy storage, sensors, and insulatingmaterials.

2,2,6,6-tetramethylpyperidine-1-oxyl (TEMPO) oxidation aided with gentlemechanical treatment could convert over 97% of pure cellulose from ricestraw into highly crystalline (60-70%) and ultrathin (1-2 nm)nanofibrils with high aspect ratios (500-1000). Freezing of TEMPOoxidized rice straw cellulose nanofibrils (CNFs) suspension at −20° C.followed by freeze-drying yielded honeycomb structured porous aerogelwith ultra-high water absorption capacity of 116-210 g/g, excellent wetresiliency to fully recover to its original shape after 100compression-recovery cycles and super amphiphilicity to absorb not onlywater but also non-polar liquids (up to 375 g/g chloroform). However,the major drawback of these CNF aerogels are their weak dry strength(Young's modulus of 54 kPa at 8.1 mg/cm³ density) even though itincreases modestly with increasing densities. While CNF aerogels areamphiphilic super absorbents, hydrophobization via atomic layerdeposition of TiO₂ or vapor deposition of organosilane tunes theabsorbency to selectively favor nonpolar liquids to favor applicationssuch as oil-water separation. Neither of surface modification alters themechanical properties of the aerogels.

The goal of this work was to simultaneously improve the mechanicalproperties and tune the amphiphilicity of CNF aerogels and cellulosesubmicron fiber aerogels. The approaches described include modificationof physical and chemical properties (density, porosities, amphililicity,hydrophilicity, hydrophobicity, surfaces charges) to simultaneouslyimprove absorption capacity, mechanical properties and hydrophobicity ofCNF aerogels and cellulose submicron fiber aerogels.

BRIEF SUMMARY OF THE INVENTION

In one embodiment, the present invention provides a method for preparingan aerogel or a foam, the method comprising forming a reaction mixturecomprising a cellulose nanofibril gel, a first solvent, and one or morecrosslinking agents under conditions sufficient to crosslink the gel;and contacting the crosslinked gel with a second solvent underconditions sufficient to dry the crosslinked gel, thereby forming anaerogel or foam.

In another embodiment, the present invention provides a method forpreparing an aerogel or a foam, the method comprising: freezing anaqueous suspension comprising cellulose nanofibrils, cellulosesub-micron fibers, or a combination thereof; freeze-drying the frozensuspension, thereby forming an aerogel or foam; and at least one of thefollowing: (1) contacting the aerogel or foam with an organosilane underconditions sufficient to deposit the organosilane onto the aerogel orfoam; or (2) heating the aerogel or foam under conditions sufficient tocarbonize the aerogel or foam.

In another embodiment, the present invention provides an aerogel or foamprepared by the method comprising forming a reaction mixture comprisinga cellulose nanofibril gel, a first solvent, and one or morecrosslinking agents under conditions sufficient to crosslink the gel;and contacting the crosslinked gel with a second solvent underconditions sufficient to dry the crosslinked gel, thereby forming anaerogel or foam.

In another embodiment, the present invention provides an aerogel or foamprepared by the method comprising: freezing an aqueous suspensioncomprising cellulose nanofibrils, cellulose sub-micron fibers, or acombination thereof; freeze-drying the frozen suspension, therebyforming an aerogel or foam; and at least one of the following: (1)contacting the aerogel or foam with an organosilane under conditionssufficient to deposit the organosilane onto the aerogel or foam; or (2)heating the aerogel or foam under conditions sufficient to carbonize theaerogel or foam.

In another embodiment, the present invention provides a supercapacitorelectrode comprising the aerogel or foam prepared by the methodcomprising: freezing an aqueous suspension comprising cellulosenanofibrils, cellulose sub-micron fibers, or a combination thereof;freeze-drying the frozen suspension, thereby forming an aerogel or foam;and at least one of the following: (1) contacting the aerogel or foamwith an organosilane under conditions sufficient to deposit theorganosilane onto the aerogel or foam; or (2) heating the aerogel orfoam under conditions sufficient to carbonize the aerogel or foam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A-1C show the appearance of HCl and FT CNFs. FIG. 1A shows theappearance of HCl (left) and FT (right) hydrogels. FIG. 1B shows theappearance of HCl (left) and FT (right) aerogels. FIG. 1C shows thevisible light transmittance of aq. 0.6% CNFs and hydrogels (2 mm thick).

FIG. 2A-2F show the SEM images of FT-CNF and HCl-CNF aerogels. FIG. 2Ashows an SEM image of FT-CNF aerogels when the scale bar is 500 μm. FIG.2B shows an SEM image of FT-CNF aerogels when the scale bar is 2 μm.FIG. 2C shows an SEM image of FT-CNF aerogels when the scale bar is 500nm. FIG. 2D shows an SEM image of HCl-CNF aerogels when the scale bar is500 μm. FIG. 2E shows an SEM image of HCl-CNF aerogels when the scalebar is 2 μm. FIG. 2F shows an SEM image of HCl-CNF aerogels when thescale bar is 500 nm.

FIG. 3A-3D show characteristics of FT-CNF and HCl-CNF aerogels. FIG. 3Ashows BET specific surface, inset are specific surface area and totalpore volume. FIG. 3B shows BJH pore size distribution. FIG. 3C shows aTGA spectra of FT-CNF and HCl aerogels. FIG. 3D shows compressionstress-strain curve of FT-CNF and HCl-CNF aerogels.

FIG. 4A-4F show SEM images of FT CNF-MDI aerogels at varied CNF:MDIratios. FIG. 4A shows an SEM image when the CNF:MDI ratio is 1:1 and thescale bar is 50 μm. FIG. 4B shows an SEM image when the CNF:MDI ratio is1:2 and the scale bar is 50 μm. FIG. 4C shows an SEM image when theCNF:MDI ratio is 1:4 and the scale bar is 50 μm. FIG. 4D shows an SEMimage when the CNF:MDI ratio is 1:1 and the scale bar is 2 μm. FIG. 4Eshows an SEM image when the CNF:MDI ratio is 1:2 and the scale bar is 50μm. FIG. 4F shows an SEM image when the CNF:MDI ratio is 1:4 and thescale bar is 50 μm.

FIG. 5A-5B show spectra of pure and MDI crosslinked FT-CNF aerogel. FIG.5A shows a FTIR spectra. FIG. 5B shows a TGA spectra.

FIG. 6A shows compressive stress-strain curves of pure and MDIcrosslinked FT-CNF aerogels. FIG. 6B shows log-log plots of Young'smodulus. FIG. 6C shows log-log plots of yield stress. FIG. 6D showslog-log plots of ultimate stress versus density of aerogels.

FIG. 7A shows BET nitrogen adsorption-desorption isotherm of pure andMDI crosslinked FT-CNF aerogel. FIG. 7B shows pore size distribution ofpure and MDI crosslinked FT-CNF aerogel.

FIG. 8A shows water (dyed in blue with methylene blue) and chloroform(dyed in red with Sudan IV red) absorption onto FT-CNF aerogel. FIG. 8Bshows water (dyed in blue with methylene blue) and chloroform (dyed inred with Sudan IV red) absorption onto CNF1MDI aerogel. FIG. 8C showswater (dyed in blue with methylene blue) and chloroform (dyed in redwith Sudan IV red) absorption onto CNF2NDI aerogel. FIG. 8D shows water(dyed in blue with methylene blue) and chloroform (dyed in red withSudan IV red) absorption onto CNF4MDI aerogel. FIG. 8E shows absorptioncapacity of CNF aerogels toward water and chloroform. FIG. 8F showscyclic absorption of chloroform onto CNF aerogels.

FIG. 9A-9E show oil-water separation using CNF4MDI as a filter. FIG. 9Ashows phase-separated 1:1 v/v water-chloroform (dyed in red). FIG. 9Bshows filtration setup with the CNF4MDI aerogel placed between the clap,water retained above and the denser chloroform passed through into theflask below. FIG. 9C shows purified water after filtration. FIG. 9Dshows CNF4MDI before filtration. FIG. 9E shows CNF4MDI after filtration.

FIG. 10A-10B show light microscopy images of ES cellulose. FIG. 10Ashows ES cellulose membrane pieces before blending. FIG. 10B shows EScellulose membrane pieces after blending.

FIG. 11A shows 0.1-0.6 wt % of ultra-fine ES cellulose fibers. FIG. 11Bshows the ES cellulose fibers densities and porosities.

FIG. 12 shows ES cellulose aerogels after redispersion in water by lightmicroscopy image.

FIG. 13A-13B show absorption and density of ES cellulose fibers. FIG.13A shows the absorption capacity and density. FIG. 13B shows theabsorption ratio and density.

FIG. 14A-14H show SEM images of the ES cellulose aerogels. FIG. 14Ashows an image of 0.1% wt of the ultra-fine ES cellulose aerogel. FIG.14B shows an image of 0.2% wt of the ultra-fine ES cellulose aerogel.FIG. 14C shows an image of 0.3% wt of the ultra-fine ES celluloseaerogel. FIG. 14D shows an image of 0.4% wt of the ultra-fine EScellulose aerogel. FIG. 14E shows an image of 0.5% wt of the ultra-fineES cellulose aerogel. FIG. 14F shows an image of 0.6% wt of theultra-fine ES cellulose aerogel. FIG. 14G shows an image of the originalES membrane. FIG. 14H shows an image of an assembled 0.1% ES aerogel.

FIG. 15A shows BET absorption-desorption of the ES cellulose aerogel.FIG. 15B shows pore size distribution of ECF aerogel.

FIG. 16 shows XRD spectra of ES cellulose membrane and aerogel.

FIG. 17A shows that a piece of ES cellulose aerogel could withstand a 2oz weight, or over 2250 times of its own weight. FIG. 17B shows the EScellulose aerogel recovery % in relation to the concentration. FIG. 17Cshows the loading-unloading curve of the 0.4% ES aerogel.

FIG. 18A shows SEM of silanized ECF aerogel. FIG. 18B shows EDS C (58.8at %) mapping of silanized ECF aerogel. FIG. 18C shows EDS 0 (38.3 at %)mapping of silanized ECF aerogel. FIG. 18D shows EDS Ci (2.5 at %)mapping of silanized ECF aerogel.

FIG. 19A shows the hydrophobicity of the ES cellulose aerogel with awater droplet deposited on the ES cellulose aerogel. FIG. 19B shows theES cellulose aerogel floating on the water surface.

FIG. 20A-20B show the absorption capacity of the ES cellulose aerogel inrelation to the density and the liquid used for absorption. FIG. 20Ashows the absorption capacity of ES cellulose aerogel in g/g in relationto density. FIG. 20B shows the absorption capacity of ES celluloseaerogel in mL/g in relation to density.

FIG. 21 shows the squeezing-reabsorption capacity cycle of the EScellulose aerogel.

FIG. 22A shows a cyclic voltammogram of the ES cellulose aerogel. FIG.22B shows a Nyquist plot of the ES cellulose aerogel. FIG. 22C shows theconstant current charge/discharge curves at 1 mA/cm² of carbonized ECFaerogel.

FIG. 23A-F show characterizations of CNFs in aqueous state. FIG. 23Ashows the lateral dimension distribution of CNF height. FIG. 23B showsthe lateral dimension distributions by AFM. FIG. 23C shows thecorresponding height profile along the line in FIG. 23B. FIG. 23D showsthe lateral dimension distribution of CNF width. FIG. 23E shows thelateral dimension distributions by TEM. FIG. 23F shows the effect ofprotonation on COO⁻Na⁺/COOH content (mmol/g CNF) by conductometrictitration.

FIG. 24A-24I show the effect of protonation on aerogel morphology byfreezing 0.6 wt % CNF suspensions at −20° C. followed by freeze drying.FIG. 24A shows CNFs with 11% COOH and radial cross-sections. FIG. 24Bshows CNFs with 11% COOH and longitudinal cross-sections. FIG. 24C showsCNFs with 11% COOH and external surfaces. FIG. 24D shows CNFs with 46%COOH and radial cross-sections. FIG. 24E shows CNFs with 46% COOH andlongitudinal cross-sections. FIG. 24F shows CNFs with 46% COOH andexternal surfaces. FIG. 24G shows CNFs with 100% COOH and radialcross-sections. FIG. 24H shows CNFs with 100% COOH and longitudinalcross-sections. FIG. 24I shows 100% COOH and external surfaces.

FIG. 25A-25D show the effect of protonation on cellular pore wallthickness and macropore diameter. FIG. 25A shows radial cross sectionsof 11% COOH aerogel with a scale bar of 10 μm. FIG. 25B shows radialcross sections of 11% COOH aerogel with a scale bar of 500 μm with theinset at 1 mm. FIG. 25C shows radial cross sections of 100% COOH aerogelwith a scale bar of 10 μm. FIG. 25D shows radial cross sections of 100%COOH aerogel with a scale bar of 500 μm, with the inset at 500 μm.

FIG. 26A-26C show the effect of protonation on aerogel wet-strength.FIG. 26A shows incremental compression (0.4 to 0.8 strain, loading andunloading) of 0.6 wt % CNF aerogels of 11% COOH aerogel. FIG. 26B showsincremental compression (0.4 to 0.8 strain, loading and unloading) of0.6 wt % CNF aerogels of 46% COOH aerogel. FIG. 26C shows showsincremental compression (0.4 to 0.8 strain, loading and unloading) of0.6 wt % CNF aerogels of 100% COOH aerogel. FIG. 26D shows thecorresponding maximum stress at 0.4, 0.6, and 0.8 strain as effected byprotonation.

FIG. 27A show water immersion (pH 5.7) on the compressive wet-strengthand dimensional integrity of 11% COOH aerogel with inset image showingcore fraction following 32 h compressive test. FIG. 27B shows the effectof protonation on aerogel cyclic water absorption and capacityretention. FIG. 27C shows additional absorption-squeezing of 100% COOHaerogel showing progressive loss of absorptive capacity and shaperecovery attributed to aerogel ductility

FIG. 28A shows the effect of protonation on aerogel chemistry, thermalbehavior, and recrystallization in an FTIR spectra of CNF aerogels from0.6% wt CNF suspension. FIG. 28B shows the effect of protonation onaerogel chemistry, thermal behavior, and recrystallization in an XRDspectra of CNF aerogels from 0.6% wt CNF suspension. FIG. 28C shows theeffect of protonation on aerogel chemistry, thermal behavior, andrecrystallization in an TGA spectra of CNF aerogels from 0.6% wt CNFsuspension. FIG. 28D shows the effect of protonation on aerogelchemistry, thermal behavior, and recrystallization in an dTGA spectra ofCNF aerogels from 0.6% wt CNF suspension. FIG. 28E shows tabulation ofdata showing maximum degradation temperature[s] (T_(max)), char residuesat 500° C., and crystallinity index (CrI).

FIG. 29A-29F show the effect of interfacial tube properties on 11% COOHaerogel morphology: FIG. 29A shows a SEM image of aerogel in hydrophilicborosilicate glass showing radial cross-sections. FIG. 29B shows a SEMimage of aerogel in hydrophilic borosilicate glass showing longitudinalcross-sections. FIG. 29C shows a SEM image of aerogel in hydrophilicborosilicate glass showing external surfaces. FIG. 29D shows a SEM imageof aerogel in polypropylene showing radial cross-sections. FIG. 29Eshows a SEM image of aerogel in polypropylene showing longitudinalcross-sections. FIG. 29F shows a SEM image of aerogel in polypropyleneshowing external surfaces.

FIG. 30A-30F show the effect of hydrophilic versus hydrophobic tubes on11% COOH aerogel morphology and external wetting behavior. FIG. 30Ashows aerogel from borosilicate glass mold. FIG. 30B shows aerogel fromborosilicate glass mold. FIG. 30C shows aerogel from borosilicate glassmold. FIG. 30D shows aerogel from polypropylene mold. FIG. 30E showsaerogel from polypropylene mold. FIG. 30F shows aerogel frompolypropylene mold.

DETAILED DESCRIPTION OF THE INVENTION

I. General

The present invention provides methods of making aerogels. The presentinvention provides methods of making cellulose nanofibril aerogels orcellulose sub-micron fiber aerogels, or a combination thereof. Thepresent invention also provides a supercapacitor electrode comprisingthe aerogels prepared by methods of the present invention.

II. Definitions

Unless specifically indicated otherwise, all technical and scientificterms used herein have the same meaning as commonly understood by thoseof ordinary skill in the art to which this invention belongs. Inaddition, any method or material similar or equivalent to a method ormaterial described herein can be used in the practice of the presentinvention. For purposes of the present invention, the following termsare defined.

“A,” “an,” or “the” as used herein not only include aspects with onemember, but also include aspects with more than one member. Forinstance, the singular forms “a,” “an,” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to “a cell” includes a plurality of such cells andreference to “the agent” includes reference to one or more agents knownto those skilled in the art, and so forth.

“Aerogel” refers to a porous solid material with high porosity and lowdensity. The pores of the aerogel allow for passage of gas or liquidphase molecules through the material. Representative aerogels includeinorganic aerogels (such as silicon-derived aerogels), organic aerogels(such as carbon-derived aerogels), and inorganic/organic hybridaerogels. Organic aerogels include, but are not limited to celluloseaerogels, urethane aerogels, resorcinol formaldehyde aerogels,polyolefin aerogels, melamine-formaldehyde aerogels, phenol-furfuralaerogels and polyimide aerogels.

“Foam” refers to a material formed with pockets of gas in a liquid orsolid.

“Cellulose nanofibril gel” refers to cellulose nanofibrils fabricatedinto a gel form. Suitable fabrication methods include, but are notlimited to, using aqueous cellulose nanofibril suspensions to formhydrogels to fabricate cellulose nanofibril gels.

“Cellulose nanofibrils” (CNF) refers to a type of nanocellulose whereinthe nanofibrils are formed from cellulose. The cellulose may bechemically modified or unmodified. Nanocellulose refers to a relativelycrystalline cellulose in either rod-like or fibril-like forms withnanometer scale lateral dimensions and hundreds to thousands of nm inlengths.

“Cellulose sub-micron fibers” refers to submicron-sized fibers formedfrom cellulose. The cellulose may be chemically modified or unmodified.

“Crosslinking agents” refers to compounds which interconnect two solidmaterials. A crosslinking agent is typically a bifunctional compoundwherein one or more reactive functional group reacts with one solidsupport or material and one or more reactive functional group reactswith another solid support or material, thereby linking the two solidsupport or material members to each other. Representative examples ofcrosslinking agents include, but are not limited to, diisocyanates,polymers, coated polymers, amino acids, di(meth)acrylates,bisacrylamide, and divinylbenzenes.

“Drying” refers to a dehydration process which removes nearly all theliquid and/or moisture from the material. Representative drying methodsof the present invention include, but are not limited to, freeze-drying,air-drying, supercritical-drying, vacuum drying, dielectric drying, andheating.

“Freeze-drying” refers to a low temperature dehydration process, and insome embodiments is referred to as lyopholisation or cryodesiccation.Freeze-drying refers to a process wherein liquid is removed from thematerial by first freezing the liquid in the product, and then loweringthe pressure and removing the frozen liquid by sublimation.

“Acid” refers to a compound that is capable of donating a proton (H⁺)under the Bronsted-Lowry definition, or is an electron pair acceptorunder the Lewis definition. Acids useful in the present invention areBronsted-Lowry acids that include, but are not limited to, alkanoicacids or carboxylic acids (formic acid, acetic acid, citric acid, lacticacid, oxalic acid, etc.), sulfonic acids and mineral acids, as definedherein. Mineral acids are inorganic acids such as hydrogen halides(hydrofluoric acid, hydrochloric acid, hydrobromice acid, etc.), halogenoxoacids (hypochlorous acid, perchloric acid, etc.), as well as sulfuricacid, nitric acid, phosphoric acid, chromic acid and boric acid.Sulfonic acids include methanesulfonic acid, benzenesulfonic acid,p-toluenesulfonic acid, triflouromethanesulfonic acid, among others.

“Diisocyanate” refers to a compound which comprises two isocyanatefunctional groups on the ends of the compound. Diisocyantes may reactwith polyols to form urethane linkages. Diisocyanates useful in thepresent invention include, but are not limited to, methylene diphenyldiisocyanate (MDI), toluene diisocyanate (TDI), isophorone diisocyante(IPDI), methylene dicyclohexyl diisocyante (HMDI), and mexamethelenediisocyante (HDI).

“Coated polymers” refers to polymers coated onto aerogels. Examples ofcoated polymers include soluble organic polymers, such as, but notlimited to cellulose esters.

“Organosilane” refers to organic compound derivatives which contain asilicon atom bonding to at least one carbon atom.

“Carbonize” refers to a process of conversion of a carbon-containingsource or material to form a material comprising primarily carbon.Carbonization is typically performed at high temperature.

“Electrospinning” refers to a fiber production method which useselectric force to draw charged threads of polymer solutions or polymermelts wherein the fiber diameters can be as small as some hundrednanometers.

“Hydrolysis” refers to a chemical reaction wherein water molecules areconsumed, for example, in the conversion of an ester to an acid and analcohol, or in the separation of compounds, macromolecules, polymers,biomolecules, or gels.

“Mechanical dispersion” refers to a process wherein the solutes and/orprecipitates in a solution are mechanically mixed. Mechanical mixing mayinclude, but is not limited to, high speed blending, shaking thesolution, or using a stir bar and stir plate.

“Absorption capacity” refers to the difference in weight between a fullysaturated aerogel and dry aerogel, divided by the weight of the dryaerogel. The liquid of the saturated aerogel can be an organic liquid orwater. Absorption capacity also comprises organic liquid absorptioncapacity, wherein the saturation is in an organic liquid. The formula todetermine absorption capacity is represented by formula (I):

$\begin{matrix}{{{Absorption}\mspace{14mu}{capacity}} = \frac{\left( {w_{e} - w_{o}} \right)}{w_{o}}} & (I)\end{matrix}$

wherein w_(e) and w_(o) are the weights of fully saturated and dryaerogels, respectively. The units for absorption capacity can be volumeover mass, such as mL/g, or mass over mass, such as g/g, or any unitconversion equivalent.

“Specific surface area” refers to the total surface area as calculatedin the Brunauer-Emmett-Teller (BET) method. The specific surface areadetermined by BET is the total surface area as all the porous structuresadsorb the small gas molecules. The units for specific surface area canbe area per volume, such as in m²/g, or any unit conversion equivalent.

“Young's modulus” refers to the stiffness of a solid material, and insome embodiments may be referred to as modulus of elasticity. The unitsfor Young's modulus can be in pressure, such as kPa, or any unitconversion equivalent.

“Ultimate stress” refers to the maximum stress that a material canwithstand while being stretched or pulled before breaking. The units forultimate stress can be in pressure, such as kPa, or any unit conversionequivalent.

“Porosity” refers to a measure of the void, or empty, spaces in amaterial. Porosity is a fraction of the volume of voids over the totalvolume, between 0 and 1, or as a percentage between 0% and 100%, whereinthe higher value indicates a higher porosity.

“Supercapacitor electrode” refers to an electrode used for ahigh-capacity capacitor. A capacitor is a device used to store anelectric charge, and a supercapacitor has a relatively higher energydensity compared to a common capacitor.

“Specific capacitance” refers to the amount of electric charge needed toraise the electric potential of an isolated conductor by one gram ofmaterial. The units for specific capacitance can be in electricalcapacitance per mass, such as Farad/g (F/g) or any unit conversionequivalent.

“Areal capacitance” refers to a capacitance per unit area. Capacitanceis the ability of a system to store an electric charge, and is the ratioof change in an electric charge to the change in its electric potential.The units for areal capacitance can be in electrical capacitance perarea, such as F/cm² or any unit conversion equivalent.

“Equivalent series resistance” refers to a resistive component of acapacitor's equivalent circuit. A capacitor may be modeled as an idealcapacitor in series with a resistor and an inductor. The resistor'svalue is the equivalent series resistance. In some embodiments, it maybe referred to as effective series resistance. Equivalent seriesresistance can be measured in Ohms.

III. Aerogels, Foams and Methods of Making

In some embodiments, the present invention provides a method forpreparing an aerogel or a foam, the method comprising forming a reactionmixture comprising a cellulose nanofibril gel, a first solvent, and oneor more crosslinking agents under conditions sufficient to crosslink thegel; and contacting the crosslinked gel with a second solvent underconditions sufficient to dry the crosslinked gel, thereby forming anaerogel or foam.

The cellulose nanofibril gel useful in the method of the presentinvention can be made by any method known by one of skill in the art.For example, the cellulose nanofibril gels can be made by freeze-thawingmethods. In another example, the cellulose nanofibril gels can be madeby acidifying methods.

The cellulose nanofibril (CNF) can be unprotonated, partially protonatedor completely protonated. For example, the CNFs surface carboxylates canbe protonated from 0% to 100%, or about 1% to about 99%, or about 10% toabout 90%, or about 25% to about 75%, or about 1, 2, 3, 4, 5, 10, 15,20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, or 100%.In some embodiments, the CNFs surface carboxylates are unprotonated. Insome embodiments, the CNFs surface carboxylates are partiallyprotonated. In some embodiments, the CNFs surface carboxylates aresubstantially protonated. In some embodiments, the CNFs surfacecarboxylates are completely protonated. In some embodiments, the CNFssurface carboxylates can be protonated by about 11%. In someembodiments, the CNFs surface carboxylates can be protonated by about46%. In some embodiments, the CNFs surface carboxylates can beprotonated by about 100%.

The freeze-thawing methods useful in the method of the present inventionis known by one of skill in the art. For example, the freeze thawingmethod comprises freezing a solution or suspension and then thawing thefrozen solution or suspension to form a gel. For example, aqueous CNFsuspensions can be made into cellulose nanofibril gels by freezing theaqueous CNF suspension, then thawing the frozen suspension to form afreeze-thawed cellulose nanofibril hydrogel (FT-hydrogel). TheFT-hydrogel is then exchange with acetone to form a cellulose nanofibrilgel. In one example, the freezing temperature is about −20° C. and thethawing temperature is ambient temperature.

The acidifying methods useful in the method of the present invention isknown by one of skill in the art. For example, the acidifying methodcomprises adding sufficient acid to the solution or suspension andrefrigerating it to form a gel. In one example, aqueous CNF suspensionis treated with HCl and refrigerated overnight to form an HCl cellulosenanofibril hydrogel (HCl-hydrogel). The HCl-hydrogel is then exchangedwith acetone to form a cellulose nanofibril gel.

The first solvent useful in the method of the present invention can beany solvent known by one of skill in the art. For example, the firstsolvent can be a polar or a non-polar organic solvent. Polar solventsinclude tert-butanol, ethanol, dichlormethane, ethyl acetate,dimethylformamide, dimethylsulfoxide, and acetone. Non-polar solventsinclude toluene, benzene, hexane, cyclohexane, and pentane. In someembodiments, the first solvent comprises acetone, ethanol, dimethylsulfoxide, dimethylformamide, toluene, chloroform, or a combinationthereof. In some embodiments, the first solvent is acetone.

Suitable crosslinking agents useful in the method of the presentinvention include, but are not limited to, diisocyanates, polymers, andcoated polymers. In some embodiments, the one or more crosslinkingagents comprise one or more diisocyanates, one or more coated polymers,or a combination thereof. In some embodiments, the one or morecrosslinking agents are one more diisocyantes.

The diisocyanates useful in the method of the present invention can beany suitable diisocyanates known by one of skill in the art. Suitablediisocyanates of the present invention may include, but are not limitedto, methylene diphenyl diisocyanate (MDI), toluene diisocyanate (TDI),isophorone diisocyante (IPDI), methylene dicyclohexyl diisocyante(HMDI), and mexamethelene diisocyante (HDI). In some embodiments, theone or more diisocyanates comprise methylene diphenyl diisocyanate.

Suitable polymers and coated polymers useful in the method of thepresent invention include, but are not limited to, soluble organicpolymers. Soluble organic polymers can be cellulose esters, biopolymers,or synthesized polymers. In some embodiments, the one or more coatedpolymers comprise cellulose acetate.

The crosslinking bonds useful in the method of the present invention canbe any crosslinking bond formed from suitable crosslinking agents andthe gels. For example, the crosslinking bonds can be urethane bonds,ester bonds, or ether bonds. In some embodiments, the crosslinking bondis a urethane bond.

The crosslinking conditions to form the crosslinking bonds useful in themethod of the present invention can be any suitable crosslinkingcondition known by one of skill in the art. Any suitable temperature,pressure, and time can be used. For example, the temperature can be atambient, or room temperature, or between about 20° C. to about 25° C. Insome examples, the temperature and pressure can be about standardtemperature and pressure. In some examples, the time can be about 2hours to 48 hours, about 4 hours to 48 hours, about 12 hours to 48hours, or about 24 hours to 48 hours.

A base can also be used to catalyze the reaction to form thecrosslinking bonds. For example, the base can be an amine base, such astrimethylamine. In some embodiments, the crosslinking conditions can beat room temperature, for 48 hours, with trimethylamine as catalyst.

In some embodiments, the crosslinked gel formed from the method of thepresent invention is washed following the crosslinking step. Washingconditions useful in the method of the present invention can be anysuitable washing conditions known by one of skill in the art. Forexample, washing conditions can be with any suitable solvent at anysuitable temperature for any amount of time. For example, washingconditions can be with organic solvents at any suitable temperaturewherein the crosslinked gel is washed more than once. In some examples,the temperature is ambient or room temperature. In some embodiments, thecrosslinked gel is washed with the first solvent following thecrosslinking step. In some embodiments, the crosslinked gel is washedwith acetone.

The second solvent useful in the method of the present invention can beany solvent known by one of skill in the art. For example, the secondsolvent can be a polar or a non-polar organic solvent. Polar solventsinclude tert-butanol, ethanol, dichlormethane, ethyl acetate,dimethylformamide, dimethylsulfoxide, and acetone. Non-polar solventsinclude toluene, benzene, hexane, cyclohexane, and pentane. In someembodiments, the second solvent comprises tert-butanol, hexane, or acombination thereof. In some embodiments, the second solvent istert-butanol.

Suitable drying conditions useful in the method of the present inventioninclude, but are not limited to freeze-drying, air-drying, andsupercritical-drying.

The freeze-drying conditions useful in the method of the presentinvention can be any suitable freeze-drying condition known by one ofskill in the art. For example, a freezedrier machine can be used forfreeze-drying. Any suitable temperature, time, or pressure can be usedfor freeze-drying. In some examples, the freeze-drying temperature isbetween about −60° C. to about −10° C., about −60° C. to about −20° C.,about −50° C. to about −20° C. or about −50° C. to about −10° C. In someexamples, the freeze-drying temperature is about −60° C., −50° C., about−40° C., about −30° C., about −20° C., or about −10° C. In someexamples, the freeze-drying pressuring can be about 0 mbar to about 10mbar, about 0 mbar to about 0.5 mbar, about 0 mbar to about 2 mbar, fromabout 0 mbar to about 1 mbar, or from about 0 mbar to about 0.05 mbar.In some examples, the freeze-drying pressure can be about 0.05 mbar,about 1 mbar, about 1.5 mbar, or about 2 mbar. In some examples, thetime is about 1 hour to about 24 hours, about 2 hours to about 24 hours,about 4 hours to about 24 hours, or about 6 hours to about 24 hours. Insome examples, the time is about 24 hours. In some embodiments, dryingthe crosslinked gel comprises freeze-drying at a temperature of about−50° C. and at a pressure of about 0.05 mbar.

The air-drying conditions useful in the method of the present inventioncan be any air-drying condition known by one of skill in the art. Forexample, any suitable temperature, time, or pressure can be used forair-drying. In some examples, the temperature for air drying can beabout 15° C. to about 30° C., or about 20° C. to about 25° C. In someexamples the temperature can be at least about 15° C., at least about20° C., at least about 25° C., or at least about 30° C. In someexamples, the pressure can be about atmospheric pressure. In someexamples, the time can be about 1 hour to about 24 hours, about 2 hoursto about 24 hours, about 4 hours to about 24 hours, or about 6 hours toabout 24 hours. In some embodiments, drying the crosslinked gelcomprises air drying at a temperature of at least about 20° C.

In some embodiments, the reaction mixture further comprises an acid andthe conditions further comprise conditions sufficient to protonate thegel. The acid useful in the method of the present invention can be anyBronsted-Lowry acid known by one of skill in the art. For example, theBronsted-Lowry acid can be an organic acid or an inorganic acid. In someexample, the inorganic acid can be a hydrogen halide such as HF, HCl,HBr, and HI. In some embodiments, the acid is hydrochloric acid.

The conditions sufficient to protonate the cellulose nanofibril gel inthe method of the present invention can be any sufficient conditionsknown by one of skill in the art. For example, the gel can be protonatedwith any Bronsted-Lowry acid at any suitable temperature and time. Insome examples, the temperature can be about 0° C. to about 100° C.,about 0° C. to about 50° C., about 0° C. to about 25° C., about 5° C. toabout 25° C., and about 10° C. to about 25° C. In some examples, thetime can be about 10 minutes to about 24 hours, about 1 hour to about 24hours, about 2 hours to about 24 hours, about 4 hours to about 24 hours,or about 6 hours to about 24 hours. In some examples, the gel can beprotonated with inorganic Bronsted-Lowry acids. In some embodiments, thegel can be protonated with hydrochloric acid.

In some embodiments, the present invention provides a method forpreparing an aerogel or a foam, the method comprising: freezing anaqueous suspension comprising cellulose nanofibrils, cellulosesub-micron fibers, or a combination thereof; freeze-drying the frozensuspension, thereby forming an aerogel or foam; and at least one of thefollowing: (1) contacting the aerogel or foam with an organosilane underconditions sufficient to deposit the organosilane onto the aerogel orfoam; or (2) heating the aerogel or foam under conditions sufficient tocarbonize the aerogel or foam.

The cellulose nanofibrils useful in the method of the present inventioncan be any cellulose nanofibrils known by one of skill in the art. Thecellulose nanofibrils can be modified or unmodified. For example, amodified cellulose nanofibril may contain functional groups modifiedonto the hydroxyl groups of cellulose to form esters.

The cellulose nanofibrils useful in the method of the present inventioncan be made by any method known by one of skill in the art. For example,the cellulose nanofibrils can be made from isolated pure cellulose. Insome examples, the modified cellulose nanofibrils can be made withmodified cellulose.

The method to make cellulose nanofibrils (CNFs) useful in the method ofthe present invention includes isolated pure cellulose treated undersuitable TEMPO oxidation conditions to form cellulose nanofibrils. Insome embodiments, isolated pure cellulose can be treated with TEMPO andNaClO under suitable conditions to form the cellulose nanofibrils. Insome embodiments, neutralization of cellulose treated with TEMPO andNaClO to a pH of about 7.5 produces CNFs with about 11% of total surfaceCOOH groups. In some embodiments, further protonation with 0.1 N HClproduces CNFs with about 46% to about 100% of total surface COOH groups.In some embodiments, further protonation with 0.1 N HCl produces CNFswith about 46% of total surface COOH groups, about 70% of total surfaceCOOH groups, or about 100% of total surface COOH groups.

The method to make modified cellulose nanofibrils useful in the methodof the present invention include modifying the hydroxyl functionalgroups of pure cellulose. The cellulose nanofibrils can be modified atthe hydroxyl groups of cellulose. In some examples, cellulose can befreeze-dried before modification. Freeze-dried cellulose is treated withbutadiene-sulfone under suitable conditions to modify the hydroxylgroups of cellulose to form 2,7-octadienyl ethers bonded to cellulose.Suitable conditions may include heating and Pd catalyst.

The cellulose sub-micron fibers useful in the method of the presentinvention can be any cellulose sub-micron fibers known by one of skillin the art. The cellulose sub-micron fibers can be modified orunmodified. The modified cellulose sub-micron fibers can be modified atthe hydroxyl group of the cellulose. In some embodiments, modifiedcellulose sub-micron fibers may comprise 2,7-octadienyl functionalgroups modified onto the hydroxyl groups of cellulose.

The cellulose sub-micron fibers useful in the method of the presentinvention can be made by any method known by one of skill in the art.For example, the cellulose sub-micron fibers can be made from cellulose.In some embodiments, the cellulose sub-micron fibers can be made fromcellulose acetate.

The methods to make the cellulose sub-micron fibers useful in the methodof the present invention is known by one of skill in the art. Forexample, the cellulose sub-micron fibers can be made by anelectrospinning process known by one of skill in the art. The cellulosesub-micron fibers made by the electrospinning process can formultra-fine ES cellulose fibers. In some embodiments, the cellulosesub-micron fibers can be prepared by a method comprising electrospinninga cellulose ester, hydrolysis of the electrospun ester, and mechanicaldispersion. In some embodiments, the cellulose sub-micron fibers can bemade by electrospinning cellulose acetate followed by alkalinehydrolysis.

The mechanical dispersion methods useful in the present invention isknown by one of skill in the art. For example, the mechanical dispersionmethod can be high speed blending, sonication, using a stir bar and stirplate, or shaking with hands. The high speed blending can be performedusing a machine. In some embodiments, the mechanical dispersion methodcan be shaking with hands.

The cellulose nanofibrils, cellulose sub-micron fibers, or combinationsthereof useful in the present invention can have any suitableconcentration in an aqueous suspension. In some embodiments, theconcentration of cellulose nanofibrils, cellulose sub-micron fibers, orcombination thereof in the suspension is between about 0.01% and about10% by weight. For example, the concentration can be between about 0.05%and about 0.8%, or about 0.05% and about 0.6%. In some embodiments, theconcentration can be about 0.05%, 0.1%, 0.2, 0.3%, 0.4%, 0.5%, or 0.6%.

The freezing conditions useful in the method of the present inventioncan be any suitable freezing conditions known by one of skill in theart. Any suitable temperature, time, and pressure may be used forfreezing. For example, the temperature can be between about −60° C. toabout 0° C., about −60° C. to about −10° C., about −50° C. to about 0°C., about −50° C. to about −10° C., or about −50° C. to about −20° C. Insome examples, the time can be about 1 hour to 24 hours, about 2 hoursto 24 hours, about 3 hours to 24 hours, or about 4 hours to 24 hours. Insome embodiments, aqueous suspensions of cellulose nanofibrils,cellulose sub-micron fibers, or combinations thereof can be frozen at−about 20° C. for about 4 hours. In some embodiments, the suspension isfrozen at a temperature of about −50° C.

The conditions sufficient to deposit the organosilane onto the aerogelor foam is known by one of skill in the art. Any suitable temperature,time, and pressure may be used to deposit the organosilane onto theaerogel or foam. For example, the temperature can be about 80° C. toabout 100° C., or about 80° C. to about 90° C. In some examples, thetemperature can be about 80° C., about 85° C., about 90° C., or about95° C. In some examples, the time can be about 10 minutes to 2 hours,about 10 minutes to 1 hour, or about 20 minutes to 40 minutes. In someexamples, the pressure can be from about vacuum pressure to aboutatmospheric pressure. In some embodiments, depositing the organosilaneonto the aerogel or foam comprises contacting the aerogel or foam withthe organosilane at a temperature of about 85° C. for about 30 minutesunder a vacuum.

The organosilane useful in the method of the present invention can beany suitable organosilane known by one of skill in the art. Suitableorganosilanes include, but are not limited to, methyltrichlorosilane,dichlorodimethylsilane, trimethylsilyl chloride, trichloro(hexyl)silane,butyltrichlorosilane, trichloro(octadecyl)silane,trichlor(phenyl)silane, and tolyltrichlorosilance. In some embodiments,the organosilane is methyltrichlorosilane.

The carbonizing conditions useful in the method of the present inventioncan be any suitable carbonizing condition known by one of skill in theart. Any suitable temperature, pressure and time can be used. Forexample, the temperature can be at least 600° C. or at least 700° C. Insome examples, the time can be from about 10 minutes to about 2 hours,from 10 minutes to about 1 hour, or from about 20 minutes to about 40minutes. The carbonizing conditions may be performed under gaseousatmosphere. For example, the gaseous atmosphere may be under noblegases, such as helium, neon, argon, krypton, xenon, and radon, ornitrogen gas. In some embodiments, carbonizing the aerogel or foamcomposite comprises contacting the aerogel or foam composite withnitrogen gas at a temperature of at least about 800° C. for a period ofabout 30 minutes.

In some embodiments, the present invention provides an aerogel or foamprepared by the method comprising forming a reaction mixture comprisinga cellulose nanofibril gel, a first solvent, and one or morecrosslinking agents under conditions sufficient to crosslink the gel;and contacting the crosslinked gel with a second solvent underconditions sufficient to dry the crosslinked gel, thereby forming anaerogel or foam.

The aerogel or foam useful in the present invention can be any aerogelor foam prepared by the methods of the present invention. For example,the aerogel can be formed from the cellulose nanofibril gel prepared bythe method of the present invention.

The aerogel or foam useful in the present invention can have anysuitable density known by one of skill in the art. For example, thedensity can be about 1 to about 200 mg/cm³. In some embodiments, theaerogel or foam can have a density between about 10 and 200 mg/cm³. Insome embodiments, the aerogel or foam can have a density between about10 and about 100 mg/cm³, about 10 and about 50 mg/cm³, about 20 andabout 50 mg/cm³, about 20 and about 40 mg/cm³, or about 20 and about 30mg/cm³. In some embodiments, the aerogel or foam can have a densitybetween about 1 and 20 mg/cm³, or about 1 and about 10 mg/cm³. In someembodiments, the aerogel or foam can have a density of about 6.9 mg/cm³.In some embodiments, the aerogel or foam can have a density of about 8.3mg/cm³.

The aerogel or foam useful in the present invention can have anysuitable organic liquid absorption capacity known by one of skill in theart. In some embodiments, the aerogel or foam can have an organic liquidabsorption capacity of at least about 1 to about 100 g/g. In someembodiments, the organic liquid absorption capacity can be about 40 toabout 80 g/g, about 50 to about 80 g/g, or about 60 to about 80 g/g. Insome embodiments, the organic liquid absorption capacity can be about 50mL/g to about 100 mL/g. In some embodiments, the aerogel or foam canhave an organic liquid absorption capacity of about 60 mL/g, about 80mL/g, or about 100 mL/g.

The absorbed organic liquid useful in the present invention can includeany suitable organic liquid known by one of skill in the art. Forexample, the organic liquid can be a polar or non-polar solvent. In someembodiments, the organic liquid can be chloroform.

The aerogel or foam useful in the present invention can have anysuitable specific surface area known by one of skill in the art. Forexample, suitable specific areas can be about 100 to about 700 m²/g,about 100 to about 600 m²/g, about 100 to about 500 m²/g about 100 toabout 400 m²/g, or about 100 to about 300 m²/g. In some embodiments, theaerogel or foam can have a specific surface area of at least about 100to about 230 m²/g. In some embodiments, the aerogel or foam can have aspecific surface area of at least about 120 to about 220 m²/g. In someembodiments, the aerogel or foam can have a specific surface area ofabout 209 m²/g. In some embodiments, the aerogel or foam can have aspecific surface area of about 123 m²/g. In some embodiments, theaerogel or foam can have a specific surface area of about 216 m²/g. Insome embodiments, the aerogel or foam can have a specific surface areaof about 228 m²/g.

The aerogel or foam useful in the present invention can have anysuitable Young's modulus known by one of skill in the art. In someembodiments, the aerogel or foam can have a Young's modulus betweenabout 200 and about 1,000 kPa. In some embodiments, the aerogel or foamcan have a Young's modulus between about 90 and about 1,000 kPa, betweenabout 90 and about 500 kPa, between about 90 and about 200 kPa, orbetween about 90 and about 100 kPa. In some embodiments, the aerogel orfoam can have a Young's modulus of about 94 kPa.

The aerogel or foam useful in the present invention can have anysuitable ultimate stress known by one of skill in the art. For example,the aerogel or foam can have an ultimate stress between about 25 and 500kPa. In some examples, the aerogel or foam can have an ultimate stressbetween about 25 and about 200 kPa, between about 25 and about 100 kPa,between about 30 and about 80 kPa, or between about 30 and about 70 kPa.In some embodiments, the aerogel or foam can have an ultimate stressbetween about 60 and 500 kPa. In some embodiments, the aerogel or foamcan have an ultimate stress of about 66 kPa. In some embodiments, theaerogel or foam can have an ultimate stress between about 31 and 33 kPa.

In some embodiments, the present invention provides an aerogel or foamprepared by the method comprising: freezing an aqueous suspensioncomprising cellulose nanofibrils, cellulose sub-micron fibers, or acombination thereof; freeze-drying the frozen suspension, therebyforming an aerogel or foam; and at least one of the following: (1)contacting the aerogel or foam with an organosilane under conditionssufficient to deposit the organosilane onto the aerogel or foam; or (2)heating the aerogel or foam under conditions sufficient to carbonize theaerogel or foam.

The aerogel or foam useful in the present invention can be any suitableaerogel or foam prepared by the methods of the present invention. Forexample, the aerogel can be formed from frozen aqueous suspensioncomprising cellulose nanofibrils, cellulose sub-micron fibers, or acombination thereof, prepared by the method of the present invention.

The aerogel or foam useful in the present invention can have anysuitable density known by one of skill in the art. In some embodiments,the aerogel or foam can have a density between about 1 and 200 mg/cm³.In some embodiments, the density can be between about 5 and 200 mg/cm³,about 5 and about 100 mg/cm³, about 5 and about 50 mg/cm³, about 5 andabout 15 mg/cm³, or about 5 and about 10 mg/cm³. In some embodiments,the aerogel or foam can have a density of about 6.9 mg/cm³. In someembodiments, the aerogel or foam can have a density of about 8.3 mg/cm³.

The aerogel or foam porosity useful in the present invention can haveany suitable porosity known by one of skill in the art. In someembodiments, the aerogel or foam can have a porosity of at least about90-99.9%. In some embodiments, the aerogel or foam can have a porosityof at least about 95-99.9% or at least about 97-99.9%. In someembodiments, the aerogel or foam can have a porosity of at least about99.6-99.9%.

The aerogel or foam absorption capacity useful in the present inventioncan have any suitable absorption capacity known by one of skill in theart. In some embodiments, the aerogel or foam can have an absorptioncapacity of at least about 100 to 400 g/g. In some embodiments, theaerogel or foam can have an absorption capacity of about 201 to 373 g/g,of about 120 to 283 g/g, or about 150 to 192 mL/g.

In some embodiments, the present invention provides a supercapacitorelectrode comprising the aerogel or foam prepared by the methodcomprising: freezing an aqueous suspension comprising cellulosenanofibrils, cellulose sub-micron fibers, or a combination thereof;freeze-drying the frozen suspension, thereby forming an aerogel or foam;and at least one of the following: (1) contacting the aerogel or foamwith an organosilane under conditions sufficient to deposit theorganosilane onto the aerogel or foam; or (2) heating the aerogel orfoam under conditions sufficient to carbonize the aerogel or foam.

The supercapacitor electrode of the present invention can be anysuitable supercapacitor electrode known by one of skill in the art. Forexample, the supercapacitor electrode can be constructed using twoidentical electrodes with cellulose filter paper as a separator intosymmetric button cells. The two identical electrodes can be sealed witha manual crimper. The supercapacitor electrode can comprise the aerogelof the present invention. In some embodiments, the supercapacitor cancomprise the aerogel of the present invention, and nickel foam.

The specific capacitance useful in the present invention can have anysuitable specific capacitance known by one of skill in the art. Forexample, the specific capacitance can be at least about 50 F/g(Farad/gram), at least about 75 F/g, at least about 100 F/g, or at leastabout 125 F/g. In some embodiments, the specific capacitance is at leastabout 100 F/g.

The areal capacitance useful in the present invention can have anysuitable areal capacitance known by one of skill in the art. Forexample, the areal capacitance can be at least about 25 mF/cm², at leastabout 30 mF/cm², at least about 35 mF/cm², at least about 40 mF/cm², atleast about 45 mF/cm², at least about 50 mF/cm², at least about 55mF/cm², or at least about 60 mF/cm². In some embodiments, the arealcapacitance is at least about 50 mF/cm².

The equivalent series resistance useful in the present invention canhave any suitable equivalent series resistance known by one of skill inthe art. For example, the equivalent series resistance can be betweenabout 1 and about 10 Ohms. In some embodiments, the equivalent seriesresistance can be between about 2 and about 8 Ohms, or between about 3and about 6 Ohms. In some embodiments, the equivalent series resistanceis about 4.2 Ohms.

IV. EXAMPLES Example 1. CNF Crosslinked Aerogels

Methods

Materials.

Pure cellulose was isolated from rice straw to 36% yield by a three-step2:1 toluene/ethanol extraction, acidified NaClO₂ (1.4%, pH 3-4, 70° C.,6 h) and KOH (5%, 70° C., 2 h) isolation process reported previously(Lu, P.; Hsieh, Y. L., Carbohydrate Polymers, 2012, 87, 564-573).Cellulose nanofibrils (CNFs) were derived from the isolated purecellulose employing 5 mmol/g NaClO/cellulose at pH 10, then neutralizedto pH 7 by adding 0.5 M NaOH, followed by mechanical blending (Vitamix5200) at 37,000 rpm for 30 min (Jiang, F.; Han, S.; Hsieh, Y.-L., RSCAdvances, 2013, 3, 12366-12375). Hydrochloric acid (1N, Certified,Fisher Scientific), Acetone (Histological grade, Fisher Scientific),tert-butanol (Certified, Fisher Scientific), Methylene diphenyldiisocyanate (98%, Sigma Aldrich), triethylamine (99.7%, extra pure,Sigma Aldrich), chloroform (HPLC grade, EMD), methylene blue (Certifiedbiological stain, Fisher Scientific) and Sudan IV red (Allied Chemical)were used as received without further purification. All water used waspurified using a Milli-Q plus water purification system (MilliporeCorporate, Billerica, Mass.).

Cellulose Nanofibril Hydrogels and Acetone Gels.

Aqueous CNF suspensions (0.6%) were fabricated into hydrogels by eitherfreezing-thawing (FT) or hydrochloric acid (HCl) gelation. FT hydrogelswas formed by freezing CNF aqueous suspension at −20° C. for 4 hr, thenthawing at ambient temperature. HCl hydrogel was obtained by adding 1 mLHCl (1 N) on top of 8 mL CNF aqueous suspension under static state in arefrigerator (4° C.) for overnight. Both FT and HCl hydrogels wereimmersed in HCl (0.2 N) to further protonate the surface carboxyls forenhanced gelation, then exchanged with acetone to acetone gels.

Crosslinking with Diisocyanate.

For crosslinking, the CNF acetone gels were placed in 80 mL acetonecontaining MDI at 1:1, 2:1 and 4:1 MDI:CNF mass ratio with 20 μltriethylamine as a catalyst at room temperature for 48 h. Afterreaction, the reaction solution turned to turbid with white precipitate,and the crosslinked CNF acetone gels were washed thoroughly with acetoneto remove unreacted reagents. Both uncrosslinked and crosslinked CNFacetone gel were further exchanged to tert-butanol and then freeze-dried(−50° C., 0.05 mbar) in a freezedrier (FreeZone 1.0 L Benchtop FreezeDry System, Labconco, Kansas City, Mo.). The CNF aerogels from FT andHCl gelation were designated as FT-CNF and HCl-CNF aerogel, and thosecrosslinked at 1:1, 2:1 and 4:1 MDI:CNF ratios were designated asCNF1MDI, CNF2MDI and CNF4MDI aerogels, respectively.

Characterization.

The optical transmittance of 0.6% CNF suspension, FT- and HCl-CNFhydrogels (2 mm thick) was recorded from 350 to 800 nm using Evolution600 UV-Vis spectrophotometer. The density of all CNF aerogels wascalculated based on the dimension (length and diameter) and mass of apiece of cylindrical aerogel, as measured using a digital caliper andbalance to 0.01 mm and 0.1 mg resolution, respectively. The liquidcontact angles on uncrosslinked and crosslinked FT-CNF aerogel werevisualized by dropping 10 μL of water (dyed with methylene blue) orchloroform (dyed with Sudan IV) on aerogel surface. The absorptioncapacity of CNF aerogels toward water and chloroform was measured byimmersing aerogel into 20 mL liquid and allowed to saturate, and thesurface liquid was blotted with filter paper and weighed. The absorptioncapacity (g/g) was calculated as:

$\begin{matrix}{{{Absorption}\mspace{14mu}{capacity}} = \frac{\left( {w_{e} - w_{o}} \right)}{w_{o}}} & (I)\end{matrix}$Where w_(e) and w_(o) are weights of fully saturated and dry aerogels,respectively.

The cyclic absorption capacity of uncrosslinked and crosslinked FT-CNFaerogels toward chloroform were determined by completely evaporating thepreviously absorbed chloroform in air and then re-absorbing followingthe previous method. CNF aerogel was cut along the cross sections with asharp razor, mounted with conductive carbon tape, sputter coated withgold and imaged by a field emission scanning electron microscope(FE-SEM) (XL 30-SFEG, FEI/Philips, USA) at a 5 mm working distance and5-kV accelerating voltage. FTIR spectra of CNF aerogels as transparentKBr pellets (1:100, w/w) were obtained from a Thermo Nicolet 6700spectrometer. The spectra were collected at ambient condition in thetransmittance mode from an accumulation of 128 scans at a 4 cm⁻¹resolution over the regions of 4000-400 cm⁻¹. TGA analyses of CNFaerogels were performed on a TGA-50 thermogravimetric analyzer(Shimadzu, Japan).

Each sample (5 mg) was heated at 10° C./min from 25° C. to 500° C. underpurging N₂ (50 mL/min). The specific surface area and porecharacteristics of CNF aerogels were determined by N₂ adsorption at 77 Kusing a surface area and porosity analyzer (ASAP 2000, Micromeritics,USA). Approximately 0.1 g of each sample was degassed at 35° C. for 24h. The specific surface area was determined by theBrunauer-Emmett-Teller (BET) method from the linear region of theisotherms in the 0.06-0.20 relative P/P₀ pressure range. Pore sizedistributions were derived from desorption branch of the isotherms bythe Barrett-Joyner-Halenda (BJH) method. The total pore volumes wereestimated from the amount adsorbed at a relative pressure of P/P₀ of0.98. Compressive tests were performed on 10 mm long cylindrical CNFaerogels using Instron 5566 equipped with a 5 kN load cell and twoflat-surface compression stages. The loading compressive rates were setto the same constant 1 mm/min. Young's modulus was determined from theinitial slope of σ-ε curve. The yield stress (σ_(y)) was determined atthe end of elastic region and the ultimate stress (σ_(u)) was determinedat strain (ε)=0.8. Oil-water separation was investigated using CNF4MDIaerogel (5 mm thick) in between a vacuum suction filtration device. Aphase-separated mixture of water/chloroform (200 mL, 50:50 v/v) solutionwas poured on top of aerogel membrane and filtered without pulling anyvacuum. This filtration-assisted separation was repeated for up to 10times to investigate the reusability.

Results and Discussion

CNF Aerogels Characterization.

The coupled TEMPO mediated oxidation and mechanical defibrillationproduced 1-2 nm thick and 500-1000 nm long cellulose nanofibrils (CNFs)that contain 1.29 mmol/g surface C6 carboxylate groups, of which 86% aresodium carboxylate (Jiang, F.; Han, S.; Hsieh, Y.-L., RSC Advances,2013, 3, 12366-12375). While the electrostatic repulsion among thenegatively charged surface carboxylates keeps CNFs suspended in aqueousmedia over time, aqueous CNF suspensions become viscous at as low as0.6% concentration due to the inter-fibril hydrogen bonding among theabundant surface C2, C3 and remaining C6 hydroxyls and protonatedcarboxyls and the entanglement among high aspect ratio flexible fibrils.To induce gelation of aqueous CNFs at such a low concentration,inter-CNF association was promoted by two external stimuli, i.e., slowfreezing to increase local CNF concentrations to enhance inter-CNFassociation followed by thawing, designated as freeze-thaw (FT), andprotonation with hydrochloric acid (HCl) to reduce inter-fibrilelectrostatic repulsion.

Both FT- and HCl-CNF hydrogels appeared translucent, with slightlyhigher clarity in the latter (FIG. 1A). This was corroborated by thelower visible light transmittance (10-24%) of FT-CNF hydrogel than that(31-43%) of HCl-CNF hydrogel (FIG. 1C). The much lower lighttransmittance of both CNF hydrogels, with only ⅕ thickness of lightpath, than the original 0.6% CNF suspension was clear evidence ofgreater CNF association induced by both slow freezing and reducedrepulsion. For aqueous CNF suspension, the much lower 33% lighttransmission at 350 nm than the 84% at 800 nm was likely from greaterscattering at wavelength closer to the size of CNFs as in aqueouscolloidal systems. Both FT- and HCl-CNF hydrogels were solvent exchangedwith acetone and tert-butanol, then freeze-dried, producing white opaqueaerogels with respective densities of 6.9 and 8.3 mg/cm₃ (FIG. 1B). Inthe sequential solvent exchange with acetone and tert-butanol, neithergels exhibited dimensional changes from the original hydrogels,suggesting no major impact on the associated CNF structure. However, thesolvent exchanged FT-CNF aerogel had slightly lower density than the 8.1mg/cm³ density of that from only freezing (−20° C.) without solventexchanges of the same CNFs. One possible explanation is the loss of someloosely bound CNFs during the solvent exchange steps.

The two aerogels had grossly different morphologies. FT-CNF aerogelshowed a cellular structure of biomodally distributed very large 200-500μm wide irregularly shaped honeycomb-like cells with thin walls ofclosely packed self-assembled CNFs with numerous ca. 50 nm wide andhundreds nm long slit-like spaces (FIG. 2A-2C). In contrast, HCl-CNFaerogel was mostly fibrillar, with inter-fibrillar spaces ranging fromhundreds nm to tens μm wide (FIG. 2D-2F). While few very fine fibrilsand film-like pieces were observed in FT-CNF and HCl-CNF aerogels,respectively, the major morphological differences between the two arethe cellular structure with distinctly different, i.e., by three ordersof magnitude, bimodal distributed pores of the former and the fibrillarstructure of the latter.

BET nitrogen adsorption-desorption behaviors of these aerogels providedfurther information on pores less than 100 nm and features to bediscerned by the high resolution SEM. Both aerogels showed type IVisotherms, typical of mesoporous materials (FIG. 3A). FT-CNF aerogelshowed an asymmetric H2 hysteresis loop between 0.6-0.8 p/p₀, inaddition to a less intensive H1 type hysteresis loop at above 0.9 p/p₀(FIG. 3B). The H2 hysteresis was attributed to pore blocking orpercolation effect in ink-bottle pore structures of the mesopores,consistent with the narrow slit width observed on the thin film walls.The primary sharp peak centered at 6 nm followed by a shallow one from20-100 nm further indicate the dominance of mesopores in FT-CNF aerogel.The HCl-CNF aerogel showed an H1 hysteresis of nearly paralleladsorption and desorption branches at p/p₀ above 0.9 and a small peak at8 nm followed by a much larger and broader peak centered at 62 nm,indicating significantly more macropores than mesopores, also consistentwith SEM observation. As expected, the fibrillar HCl-CNF aerogels hadhigher specific surface (209 m²/g) and pore volume (0.96 cm³/g) than themuch better self-assembled FT-CNF aerogel (123 m²/g and 0.37 cm³/g,respectively).

Both FT- and HCl-CNF aerogels exhibited similarly thermal behaviors,i.e., similarly hygroscopic, containing 7-8.2% moisture, losing 75% ofmass from 200 to 345° C. and yielding 9.1 and 3.6% chars at 500° C.,respectively (FIG. 3C). While the extent of char from HCl-CNF aerogelwas similar to native cellulose, the nearly tripled amount of char fromFT-CNF aerogel was attributed to its much extensively assembled andpacked filmlike structures and lower specific surface to heat exposure.Both aerogels were flexible and could be compressed to up to 0.8 strainwhile remaining physically intact, showing three stress-strain regionsof initial linear elastic, then plastic deformation and the finaldensification (FIG. 3D). FT-CNF aerogel showed a steeper linear elasticdeformation region with more than doubled Young's modulus of 94 kPa butless than half 0.05 yield strain as compared to the 44 kPa Young'smodulus and 0.13 yield strain for HCl-CNF aerogel. While both aerogelsshowed similar 31-33 kPa ultimate stress, FT-CNF aerogel hadsignificantly higher specific Young's modulus (13.6 MPa/g·cm³) andultimate stress (4.5 MPa/g·cm³) than HCl-CNF aerogel (5.3 and 4.0MPa/g·cm³, respectively) due to its well-assembled CNF walls in aninterconnected honeycomb structure.

When normalized by density, the specific Young's modulus and ultimatestress of FTCNF aerogel are 13.6 MPa/g·cm³ and 4.5 MPa/g·cm³,respectively, over three times higher modulus than other nanocellulosebased aerogels from freezing and freeze-drying, such as silylatedaerogel from homogenized oat straw nanocellulose frozen at −196° C. (4.1MPa/g·cm³ specific Young's modulus at 6.7 mg/cm³) (Zhang, Z.; Sebe, G.;Rentsch, D.; Zimmermann, T.; Tingaut, P., Chemistry of Materials, 2014,26, 2659-2668), ca, 70% higher modulus than aerogel from enzymaticallydegraded and homogenized softwood pulp microfibrillated cellulose frozenat −196° C. (8 MPa/g·cm³ specific Young's modulus at 7 mg/cm³)(Sehaqui,H.; Salajkova, M.; Zhou, Q.; Berglund, L. A., Soft Matter, 2010, 6,1824-1832) or −180° C. (8-9 MPa/g·cm³ specific Young's modulus atdensity of 20-30 mg/cm³)(Paakko, M.; vapaavuori, J.; Silvennoinen, R.;Kosonen, H.; Ankerfors; Lindstrom, T.; Berglund, L. A.; Ikkala, O. SoftMatter, 2008, 4, 2492-2499), 44 times ing modulus of glutaraldehydecrosslinked TEMPO oxidized eucalyptus pulp frozen at −78° C. (<0.3MPa/g·cm³ specific ultimate stress at 0.8 strain),₃₉ but similar toTEMPO oxidized softwood nanocellulose aerogel unidirectionally frozen at15 K/min (<13.5 MPa/g·cm³ specific Young's modulus at 5.6 mg/cm³)(Wicklein, B.; Kocjan, A.; Salazar-Alvarez, G.; Carosio, F.; Camino, G.;Antonietti, M.; Bergstrom, L.; Nature Nanotechnology, 2015, 10,277-283). Since the latter frozen at a slow 15 K/min rate also consistedof honeycomb like structure, the higher modulus in both cases wereconsistent with more extensive assembling of CNFs into well packed thinwall in the honeycomb structure.

FT-CNF aerogel absorbed 112.1 and 100.8 mL/g of water and chloroformrespectively, without appreciable deformation, indicating amphiphilicitywith slightly higher hydrophilicity. In contrast, HCl-CNF aerogel shrankimmediately upon exposure to the same liquids, releasing trapped airbubbles, therefore, absorbing only 28.0 and 30.2 mL/g water andchloroform, respectively, only a third of those by the FT-CNF aerogel.The shrinking of HCl-CNF aerogel was attributed to the fibrousstructure, as observed by SEM, which collapsed from the surface tensionof absorbed liquid. Based on the higher mechanical strength and bettershape retention with absorbed liquids, only FT-CNF aerogel was furthercrosslinked with MDI.

MDI Crosslinking of CNF Aerogel.

FT-CNF hydrogels were solvent exchanged into acetone gels, thencrosslinked with MDI using triethyleamine as a catalyst. Based on the4.19 mmol/g surface hydroxyls and 1.29 mmol/g carboxyls for CNFs, the1:1, 1:2 and 1:4 CNF/MDI w/w ratios were calculated to be 1:1.45, 1:2.90and 1:5.80 OH/COOH:NCO molar ratios for CNF1MDI, CNF2MDI, and CNF4MDI,respectively, all with excess MDI crosslinker. Neither acetone exchangenor MDI crosslinking caused dimensional changes but the crosslinkedacetone gels became more opaque (FIG. 51). All MDI-crosslinked acetonegels were solvent exchanged with tert-butanol and then freeze-dried intowhite MDI-crosslinked aerogels, appearing essentially the same as theuncrosslinked aerogels, however, in higher densities of 8.2, 9.8 and11.5 mg/cm₃ for FT-CNF1MDI, CNF2MDI and CMF4MDI, respectively, or 19, 42and 67% density increases.

MDI crosslinked FT-CNF aerogels retained similar honeycomb structures asthe unmodified one but with larger and more aggregates within the poresat higher MDI:CNF ratios. In contract to the smooth pore wall for theunmodified CNF aerogel (FIG. 2B), the pore walls of MDI crosslinkedaerogels were more fully covered with aggregates that increased withincreasing MDI ratios (FIG. 4A-4C), indicating more extensively reactedMDI with CNF. Closer inspection on the pore walls showed that the tensof nm slit-like spaces were mostly retained after crosslinking (FIG.4D-4F). However, it is also shown that MDI aggregates could fuse intothe spaces and anchor onto the pore wall, which was more prominent athigher MDI ratios.

Crosslinking between MDI and the hydroxyls on FT-CNF aerogel wasconfirmed by the presence of urethane link as evident by the appearanceof new peaks at 1540.8 and 1234.2 cm⁻¹, corresponding to the C—Nstretching and N—H bending of amide II and III, respectively. Thearomatic skeletal vibration at 1511.9 and 1598.7 cm⁻¹ increased withincreasing MDI crosslinking agent, showing increasing presence of MDI. Anew peak at 2270 cm⁻¹ ascribing to the vibration of N═C═O (isocyanate)in MDI also appeared on the crosslinked aerogels and increased inintensity with increasing MDI:CNF ratios, showing increasing presence ofunreacted isocyanates. However, the unreacted isocyanate peaks did notintensify to the same extents as the urethane nor the aromatic skeletal,indicating MDI participating more in crosslinking than not, butnevertheless offering reactive sites for introducing otherfunctionalities on the aerogel surfaces.

Reacting CNF aerogel with MDI reduced moisture contents (determined at150° C.) from 8.2% for FT-CNF to 3.8, 4.2 and 2.3% for CNF1MDI, CNF2MDIand CNF4MDI, respectively, significantly reducing the hygroscopicity by50% for CMF1MDI and CNF2MDI and over 70% for CNF4MDI. TheMDI-crosslinked aerogels decomposed at a considerably lower rate of−0.45%⊕° C.⁻¹ than −0.79%⊕° C.⁻¹ for FT-CNF, but in the same 200-350° C.range (FIG. 5B), retaining higher mass at the end of the primary stageof decomposion, i.e., 350° C. The derivatives of these TGA curves showeda peak at 268° C. ascribed to degradation of carboxylated CNF surfacechains while the primary degradation peak centered at around 321° C. forFT-CNF, CNF1MDI and CNF2MDI and slightly higher 331° C. for CNF4MDI.Most significantly, the char residues significantly increased to 34.7,36.1 and 43.3% with increasing MDI ratios from the 9.1% for FT-CNF. Themore than tripled and quadrupled char residues could be ascribed to thearomatic moiety and low oxygen content in MDI, and along with lowerdecomposition rates attest to improved thermal stability of MDIcrosslinked CNF aerogel.

The MDI crosslinked FT-CNF aerogels exhibited significantly increasedmodulus from 94 KPa for FT-CNF aerogel to 127, 204 and 209 MPa withincreasing MDI ratios (FIG. 6A), clearly evident of strengthening viaMDI crosslinking. Both yield stress and strain increased as the MDIratio increased, up to more than tripled and doubled for CNF4MDIaerogel, respectively. However, elastic deformation could only sustainfor up to 0.1 compressive strain for CNF4MDI, followed by a largeplastic deformation, leading to non-recovery compression. The ultimatestress also increased with increasing MDI, doubling to 66 kPa forCNF4MDI aerogel.

Since the aerogel density also increased with the MDI crosslinking,Young's modulus (E), yield stress (σ_(y)) and ultimate stress (σ_(u))were plotted against their density (ρ) in log-log scale (FIG. 6B-6D),and fitted with a power law expression,E or σ˜ρ^(n)where n is the scaling factor. All mechanical properties increased withincreasing densities, with greater than 1 scaling factor n, showingnon-linear relationships. The scaling factors for Young's modulus, yieldstress and ultimate stress of MDI crosslinked FT-CNF aerogels weredetermined to be 1.69, 2.49 and 1.43, respectively. This is in contrastto the 1 scaling factor, or no scaling law effect, previously observedfor TEMPO oxidized cellulose nanofibrils aerogel at density from 4-40mg/cm³ (Kobayashi, Y.; Saito, T.; Isogai, A., AngewandteChemie-International Edition, 2014, 53, 10394-10397). Therefore, thescaling law effect was attributed to the crosslinking of CNFs with therigid MDI molecules, imposing greater improvement in mechanicalproperties over slightly increased aerogel density.

MDI-crosslinked FT-CNF aerogels showed similarly type IV BET nitrogenadsorption-desorption isotherms, with the typical H1 hysteresis atp/p_(o) above 0.7 (FIG. 7A). With increasing MDI, the crosslinked FT-CNFaerogels shift the bimodal pore distribution of the FT-CNF aerogel,i.e., a sharp peak at 6 nm and a shallow broad peak centered at 60 nm,to increasing intensities and sizes (9.4, 10.6 and 12.0 nm for CNF1MDI,CNF2MDI and CNF4MDI, respectively) of the smaller pore to essentiallymonodistributed pore size of CNF4MDI aerogel (FIG. 7B). Both specificsurface area and pore volume increased from respective 123 m²/g and 0.37cm³/g for the FT-CNF aerogel to 216 m²/g and 0.87 cm³/g for CNF1MDIaerogel, and 228 m²/g and 1.00 cm³/g for CNF2MDI aerogel. The almostdoubled specific surface area and tripled pore volume of CNF2MDI aerogelindicated that additional smaller mesopore were created with the MDIcrosslinking, either within the MDI aggregates or in the interspacesbetween MDI and CNF cellular wall surfaces. The high specific surfacearea and pore volume is consistent with the SEM observation that showsthe tens of nm wide slit-like spaces were well preserved aftercrosslinking. At the highest MDI, CNF4MDI aerogel did not show furtherincreased specific surface, but slightly decreased the pore volume to0.94 cm³/g. Silica aerogel crosslinked with diisocyanate has also showedimproved mechanical properties and hydrophobicity, however, the originalhigh specific surface area of 700 m²/g was significantly decreased to aslow as 8 m²/g as the pores were filled with diisocyanate.

Liquid Affinity and Separation of MDI Crosslinked FT-CNF Aerogel.

FT-CNF aerogel is amphiphilic, capable of absorbing slightly more water(112.1 mL/g) than chloroform (100.8 mL/g) (FIGS. 8A and 8E). Theseliquid absorption values correspond to 77.7 and 69.9% of the porecapacity of 144.3 mL/g calculated from the aerogel density (6.9 mg/cm³)and crystalline cellulose density (1600 mg/cm³). All MDIcrosslinkedFT-CNF aerogels became clearly more hydrophobic with water beading up onthe surfaces (FIG. 8B-8D). The increasingly larger water contact angleswith higher MDI (FIG. 8B-8C) are consistent with increasing differencesbetween non-polar chloroform and polar water absorption (FIG. 8E).However, absorption of both polar and non-polar liquids decreased withincreasing crosslinking, i.e., slightly decreased chloroform absorptionto 81.7, 58.5, and 63.6 mL/g, but more sharply reduced water absorptionto 52.8, 18.1 and 5.3 mL/g for CNF1MDI, CNF2MDI and CNF4MDI,respectively. The chloroform absorption corresponded to 69.9, 67.3, 57.6and 73.7% of the pore capacities calculated from the actual densities ofthe crosslinked aerogels, i.e., of 144.3, 121.3, 101.4 and 86.3 mL/g forFT-CNF, CNF1MDI, CNF2MDI and CNF4MDI, respectively. The 70-74%chloroform absorption capacities of MDI-crosslinked CNF aerogels are infact slightly higher than the 58-70% capacities of the lighter (2.7-8.1mg/cm³ densities), suggesting higher pores accessibility.

The cyclic absorption of chloroform was investigated by evaporating theabsorbed chloroform prior to the next absorption cycle (FIG. 8F) to showdrastic decreases in absorption in the first three cycles and stabilizedafter 5 cycles. Such decreased absorption was attributed to shrinkage inboth lateral and thickness directions and the extent of shrinkagedecreased with increasing MDI ratios. The dimensional shrinkage reducedchloroform absorption as the cycles increased, lowering to 29.9, 34.2,41.5 and 57% for FT-CNF, CNF1MDI, CNF2MDI and CNF4MDI, respectively,after six cycles. In consistent with the less dimensional shrinkage, theCNF4MDI aerogel showed the highest relative absorbency, indicating thatthe MDI crosslinked structure is more rigid to resistant againstshrinkage caused by the surface tension during liquid evaporation.

Separation of oil from water was evaluated via simple filtration usingthe most hydrophobic and least hydrophilic CNF4MDI aerogel (FIG. 9A-9E).Chloroform (dyed in red color) and water were mixed at 1:1 ratios andimmediately phase separated. Upon pouring over a 5 mm thick disc CNF4MDIaerogel filter (FIG. 9D), chloroform pulled by house vacuum andpermeated through the lipophilic CNF4MDI aerogel into the flask belowwhile water was retained above by the hydrophobic CNF4MDI aerogelfilter, effectively separating water and oil. The used aerogel filterretained some chloroform as shown by the red color, but remain effectivein separating chloroform-water mixture for up to ten times. Thisrepetitive filtration capability of this MDI-crosslinked CNF aerogelfilter demonstrated clearly advantage for efficient and cyclicalseparation of large amount of oil pollutant from water, without beinglimited by the actual absorption capacity of the materials.

Conclusions

Gelation of TEMPO oxidized cellulose nanofibrils via bothfreezing-thawing and hydrochloric acid methods were explored to form CNFhydrogels that were further modified through chemical crosslinking withMDI. Freezing-thawing treatment resulted in slightly less transparenthydrogel, showing 10-24% transmittance at 350-800 nm wavelength ascompared to the 31-43% for HCl hydrogel. Solvent exchanging to acetoneand tert-butanol followed by freeze-drying yielded similarly whiteopaque aerogel with respective densities of 6.9 and 8.3 mg/cm³ for FT-and HCl-CNF aerogels. FT-CNF aerogel showed honeycomb structurecontaining both 200-500 μm wide large pores and numerous slit-likespaces of less than 50 nm on the pore walls, with 123 m²/g specificsurface area and 0.37 cm³/g pore volume. In contrast, HCl-CNF aerogelappeared mostly fibrillar structure with a few hundreds of nm widespaces among the nanoparticulates, showing higher specific surface areaand pore volume of 209 m²/g and 0.96 cm³/g, respectively. The moreextensively assembled honeycomb structure of FT-CNF aerogel led tobetter performance in terms of thermal stability (char residues of 9.1vs. 3.6% for HCl-CNF aerogel), mechanical properties (Young's modulus of94 vs. 44 kPa for HCl-CNF aerogel) and liquid absorption capacity (waterabsorption capacity of 112.1 vs. 28.0 mL/g for HCl-CNF aerogel). MDIcrosslinking of FT-CNF aerogel led to an increase in density of 19, 42,and 67% at 1:1, 2:1, and 4:1 MDI:CNF ratios, respectively. Thesuccessful crosslinking of FT-CNF aerogel with MDI was confirmed fromthe aggregates formed on the thin pore walls, as well as FTIR peaks at1540.8 and 1234.2 cm⁻¹ for C—N stretching and N—H bending of amide IIand III. MDI crosslinking clearly improved the thermal stability of CNFaerogel showing more than four times increase in the char residues, aswell as almost half degradation rate. Mechanical properties were alsosignificantly improved after crosslinking, showing power law scalingeffect vs. density with scaling factor of 1.69, 2.49 and 1.43 forYoung's modulus, yield stress and ultimate stress, respectively.Besides, both specific surface area and pore volume significantlyincreased from 123 m²/g and 0.37 cm³/g to 228 m²/g and 1.00 cm³/g aftercrosslinking. Most importantly, MDI crosslinking improved thehydrophobicity of the CNF aerogel, showing significantly reduced waterabsorption capacity from 112.1 to 5.3 mL/g. Therefore, the crosslinkedaerogel could be used as filter membrane to achieve oil-water separationvia a simple filtration method.

Example 2. Electrospun Cellulose Aerogels

Methods

Materials.

Ultra-fine cellulose fibrous membrane were prepared by electrospinningof cellulose acetate followed by alkaline hydrolysis. The as-obtainedcellulose membrane was cut into 5×5 mm pieces and then dispersed inwater by high speed blending (37,000 rpm, 5 min). Methyltrichlorosilane(99%, Sigma-Aldrich), methylene blue (Certified biological stain, FisherScientific), hexane (Certified ACS, Fisher Scientific), decane(Certified, Fisher Scientific), cyclohexane (HPLC grade, EM Science),acetone (Histological grade, Fisher Scientific), xylene (GR ACS, EMScience), toluene (Certified ACS, Fisher Scientific), pump oil (Maxima CPlus, Fisher Scientific), DMSO (GR, EMD), chloroform (Certified ACS,Fisher Scientific).

Fabrication of Electrospun (ES) Cellulose Aerogel.

ES cellulose aerogels were fabricated by freezing of aqueous suspensionsof ultrafine ES cellulose fibers (0.1-0.6 wt %) at −20° C. for 5 hfollowed by freeze-drying. Vapor deposition of methyltrichlorosilane onES cellulose aerogel (0.4%) was conducted at 85° C. for 30 min in avacuum oven. Carbonization of ES cellulose aerogel (0.6%) was conductedby heating in nitrogen at 10° C./min to 800° C. and then held at 800° C.for 30 min.

The carbonized cellulose nanofiber aerogel (CNA) was used to preparesupercapacitor electrode without binder and conductive additive. Roughly1 mg CNA was directly applied to 1 cm² nickel foam and the resultingelectrode was roller pressed to 50 μm in thickness to improve thecontact of nanofiber and nickel foam. The supercapacitor cells wereconstructed by using two identical electrodes with cellulose filterpaper as separator into symmetric button cells and sealed with a manualcrimper (CR2032, MTI). 6 M KOH in water was the electrolytes.

Results and Discussion

Blending 5 cm² ES cellulose membrane pieces generated ultra-fine fibersranged from few hundred μm to over one cm in lengths (FIG. 10A-10B).

Aqueous suspensions (0.1-0.6 wt %) of these ultra-fine ES cellulosefibers were self-assembled into super-light weight aerogels withdensities ranging from 1.1-7.0 mg/cm³ and porosity of 99.6-99.9%, viafreezing and freeze-drying (FIG. 11A-11B).

The ES cellulose aerogels were readily dispersed in water into theoriginal ultra-fine fibers by simple hand shaking as observed by thelight microscopy image (FIG. 12).

The ES cellulose aerogels are super-absorbents toward organic liquids,as demonstrated by the 201-373 g/g of absorption capacity towardchloroform, decreasing with the increased aerogel density (FIG. 13A).Based on the theoretical absorption capacity calculated from the densityof aerogel, over 95% of the pores within aerogel were accessible, muchhigher than the 70% accessible pores for aerogels assembled fromcellulose nanofibrils (CNFs).

The SEM images of the ES cellulose aerogels showed highly porousstructures with large pores of over several hundreds of microns wide(FIG. 14A-14H). Closer inspection of the pore walls indicated that theywere all assembled by the micron-sized ES fibers, and the fibers arearranged randomly similar to the original ES membrane. This is incontrast to the closely packed self-assembled cellulose nanofibrils intofilm-like structures as in the case of CNF aerogel. The micron-sized ESfibers overlay each other forming a loose network structure, whichcontribute to the more accessible pores as indicated by the liquidabsorption capacity.

BET N2 adsorption-desorption of the ES cellulose aerogel showedmacro-porous structure with specific surface area and pore volume of 7.9m²/g and 0.016 cm³/g, respectively, similar to the 7.6 m²/g and 0.015cm³/g for the original ES membrane (FIG. 15A-15B).

XRD spectra showed both ES cellulose membrane and aerogel showedsimilarly cellulose II crystalline structure (FIG. 16). Self-assembledES cellulose aerogels resembled the original ES cellulose membranes inspecific area or crystalline structure as evident by both BET and XRD.

ES cellulose aerogels showed interesting dry shape recovery propertiesin air (FIG. 17A), in contrast to the CNF aerogel that does not recoveronce compressed in air. The pictures showed that a piece of aerogel(25.2 mg) could withstand a 2 oz weight, or over 2250 times of its ownweight. Doubling the weight to 4 oz deformed the aerogel, but theaerogel recover to 86% of its original dimension upon removal of theload (FIG. 17B). The loading-unloading curve of the 0.4% ES aerogelshowed that the aerogel starts to recover once the loading is releasedand the stress does not return to 0 until the strain recover to 0.1,indicating good shape recovery properties (FIG. 17C).

The ES cellulose aerogel was modified with methyltrichlorosilane torender it hydrophobicity and retain shape when exposed to water. The EDSmapping of the modified ES cellulose aerogel showed uniformly Si atomdistribution with 2.5% atomic concentration, indicating uniform surfacereaction of cellulose fibers with methyltrichlorosilane.

The increased hydrophobicity could be clearly demonstrated by the largecontact angle between a water droplet deposited on the modified EScellulose aerogel that could stand on the water surface withoutabsorbing any water (FIG. 19A). This is completely opposite to theimmediate disintegration of the original ES cellulose aerogel whenplaced in water.

The absorption capacity of the unmodified ES cellulose aerogel rangesfrom 120-283 g/g, or 150-192 mL/g, depending on the density, viscosityand polarity of the liquid used for absorption (FIGS. 20A-20B). Forchloroform, it could be observed that the absorption capacity accountsto over 96% of the theoretical absorption capacity, indicating most ofthe pores were accessible for liquid absorption.

Besides, the modified ES cellulose aerogel showed excellent shaperecovery properties in organic solvents, enabling recovery or removingof absorbed toluene by simple squeezing and re-using of the aerogel forfurther absorption. This simple squeezing-reabsorption does lower thereabsorption capacity of the aerogel, but could maintain over 67% of itsoriginal absorption capacity in ten cycles (FIG. 21). Therefore, thisaerogel is an excellent candidate for oil removal and could be easilyrecovered.

The ES cellulose aerogel could be facilely carbonized into carbonaerogel to be used for supercapacitor. The specific capacitance of theCNA electrode was calculated to be 103 F/g and 51.3 mF/cm². The cellvoltage was set to be 1 V due to the limitation of aqueous electrolyte.The equivalent series resistance was calculated to be 4.2 Ohms based onthe Nyquist plot at 1 kHz.

Example 3. Protonated CNF

Materials and Methods

Materials.

Pure cellulose was isolated from rice straw (Calrose variety) to 38%yield by a three-step 2:1 toluene/ethanol extraction, acidified NaClO₂(1.4%, pH 3-4, 70° C., 6 h) and KOH (5%, 1 day RT, 2 hrs 90° C.)isolation process as reported previously. Hydrochloric acid (HCl, 1 N,Certified, Fisher Scientific), sodium hydroxide (NaOH, 1 N, Certified,Fisher Scientific), sodium hypochlorite solution (NaClO, 11.9%, reagentgrade, Sigma-Aldrich), 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO,99.9%, Sigma-Aldrich), sodium bromide (NaBr, BioXtra, 99.6%,Sigma-Aldrich), decane (Certified ACS, Fisher Scientific), andchloroform (HPLC grade, EMD) were used as received. All water used waspurified by Mili-Q plus water purification system (Milipore Corporate,Billerica, Mass.).

CNFs Isolation and Protonation.

Cellulose nanofibrils (CNFs) were isolated from pure rice strawcellulose via TEMPO-mediated oxidation followed by mechanical blending.Briefly, 1.0 g of rice straw cellulose was oxidized in an aqueoussolution of 0.016 g TEMPO, 0.1 g of NaBr, and 5 mmol NaClO at pH 10.0.Following termination of oxidation, the reaction mixture was neutralizedto pH 7.5 using 0.5 N HCl. After centrifugation and dialysis,TEMPO-oxidized cellulose was blended (Vitamix 5200, Vitamix Corporation)at 37 000 rpm for 30 min and centrifuged (5000 rpm, 15 min) to collectthe supernatant. Protonation of surface carboxyls was tuned by addingspecific amounts of 0.1 N HCl to dilute CNF dispersions, stirring for 30min, then followed with dialysis. All dispersions were concentratedusing a rotary evaporator (Buchi Rotavapor R-114) to 0.6 wt % and storedat 4° C. for aerogel fabrication.

Aerogel Preparation.

Aerogels were prepared as previously reported. Briefly, 0.6 wt %concentration of aq. CNF suspensions were frozen (−20° C., 15 h), thenlyophilized (−47° C., 0.05 mbar, 2 days, Free Zone 1.0 L Benchtop FreezeDry System, Labconco, Kansas City, Mo.) to yield CNF aerogels.

Characterizations.

For aqueous state CNF suspensions, a OAKTON pH/CON 510 series meter withprobe was used to measured ionic conductivity. Regarding solid stateCNFs, cylindrical aerogels were cut into 10 mm long sections. Thedimensions (length and width) and mass were measured using a digitalcaliper and balance to 0.01 mm and 0.1 mg resolution, respectively, tocalculate the aerogel density (ρ_(a), mg cm⁻³). The porosity wascalculated as:

$\begin{matrix}{{{Porosity}\mspace{14mu}(\%)} = {\left( {1 - \frac{\rho_{a}}{\rho_{c}}} \right) \times 100\%}} & (1)\end{matrix}$where ρ_(c) is the bulk density of cellulose taken as 1600 mg/cm³. Theamphiphilic liquid affinity was examined by measuring the liquidabsorption capacity (g/g) of CNF aerogels as:

$\begin{matrix}{{{Measured}\mspace{14mu}{absorption}\mspace{14mu}{capacity}} = \frac{\left( {w_{e} - w_{o}} \right)}{w_{o}\;}} & (2)\end{matrix}$where w_(e) and w_(o) are weights of fully saturated and dry aerogels,respectively. The pore volume of aerogels was calculated as:

$\begin{matrix}{{{Pore}\mspace{14mu}{volume}} = \frac{Porosity}{\rho_{a\;}}} & (3)\end{matrix}$

Aerogel pore morphology was characterized using scanning electronmicroscopy

(SEM). Aerogels were mounted on conductive carbon tape and imagingperformed on a Quattro Environmental SEM-FEG (ThermoFisher). A powderdiffractometer (PANanalytical, Malvern Panalytical) was used for wideangle X-ray diffraction (XRD) of aerogels with length of 5 mm compressedto 1 mm thick sheets between glass slides and scanned from 5 to 40° in acontinuous mode using Ni-filtered Cu Kα radiation (λ=1.5406 Å) at ananode voltage of 45 kV and a current of 40 mA. The crystallinity index(CrI) was calculated from the peak intensity of the 200 crystallineplane peak (I₂₀₀, 2θ=22.6°) and the intensity minimum between the 200and 110 (I_(am), 2θ=18.7°) peaks using the following empirical equation:

$\begin{matrix}{{CrI} = {\frac{I_{200} - I_{am}}{I_{200}} \times 100\%}} & (4)\end{matrix}$

The mechanical behavior of CNF aerogels was measured with a 5566 Instronuniversal testing machine at a constant 2.5 mm min⁻¹ compressive strainrate and 2.5 N load cell with two flat-surface compression stages.Cylindrical aerogels of 10 mm length were compressed incrementally at0.4, 0.6 and 0.8 strain.

Results and Discussion

CNF Characteristics.

Pure rice straw cellulose was defibrillated in water by one coupledTEMPO-mediated oxidation and mechanical blending to yield at nearcomplete conversion of cellulose nanofibrils (CNFs). Neutralization topH 7.5 at the end of the reaction protonated 11% of the total 1.17 mmolCOOH g⁻¹ surface carboxyl groups to free carboxyls. Further protonationwith 0.1 N HCl produced CNFs with 0.54 and or 0.18 mmol-COOH g-1 or 46and 100% surface carboxyl COOH. The dimensions of CNFs are characterizedby AFM and TEM to have average thickness of 1.38 nm±0.4, width of 2.49nm±0.3, and length of up to 1 μm.

Aerogel Structures and Morphology.

Aqueous CNFs (0.6%) were frozen (−20° C., 15 h) in borosilicate glasstubes, then freeze dried (−47° C., 0.05 mbar, 2 d) into white aerogels.With increasing protonation, the diameters of aerogels were closer tothe 14-mm ID of the glass tubes, i.e., less shrinkage, leading to lowerdensities of 8.0, 6.6 and 5.2 mg/cc, porosities as well as pore volumesof 123, 153 and 196 cc/g for 11, 46 and 100% protonated CNFs,respectively (Table 1). The relationship between pore volume andprotonation was linear (R²=0.99). These improved pore volumes are alsoreflected in increased absorption of polar (water) and non-polar(chloroform, decane) liquids. While aerogel from the more charged CNFsabsorbs more water (103 mL/g) than the non-polar (86, 78 mL/g) liquids,i.e., slightly more hydrophilic, those from the more protonated CNFsbecome more balanced amphiphilic. Increasing protonation from 11 to 46and 100% protonated also improve wet stability of aerogels for 18 h to24 h and over 32 h. All aerogels remain intact at pH 2 and in decane forat least 30 days and more stable under acidic conditions

TABLE 1 The effect or protonation on aerogel properties, aerogel shaperetention, and cell geometry and surface chemistry. Protonation 11% COOH46% COOH 100% COOH Diameter (mm)  12 ± 0.1 12.3 ± 0.1  13.1 ± 0.1 Density (mg/cc) 8.0 ± 0.6 6.6 ± 0.8 5.2 ± 0.8 Porosity (%) 99.5 ± 0.0 99.6 ± 0.1  99.7 ± 0.1  Pore volume (cc/g)  123 ± 8.5   153 ± 19.0  196± 31.1 Water absorption (mL/g)  103 ± 7.2   117 ± 17.4  166 ± 19.3Decane absorption (mL/g)  78 ± 4.3  119 ± 22.9  163 ± 29.2 Chloroformabsorption (mL/g)   86 ± 10.9  113 ± 26.1  154 ± 23.1 wet stability: pH10 18 h 24 h >32 h wet stability: pH 7 24 h 24 h >32 h wet stability: pH5.7 48 h 5 days >30 days wet stability: pH 2 >30 days >30 days >30 dayswet stability: decane >30 days >30 days >30 days

All aerogels showed isotropic pore morphologies, i.e. similarmacro-scale pores in both radial and longitudinal cross-sections (FIG.24A-24I). Closer examination by SEM of aerogel cross-sections showedslightly increasing pore diameters with increasing protonation; i.e.smaller pore diameter (207±48 μm) of that from 11% COOH to larger porediameter (227±45 μm) of the 100% COOH CNF aerogels. The cellular porewall thickness decreased with increasing protonation; i.e. from 835±371to 522±171 nm for the 11 and 100% COOH aerogels, respectively (FIG.25A-25D).

Wet Compression Strength.

All aerogels were readily and fully saturated when immersed in water andtheir wet-strength measured by cyclic loading and unloadingcompressions. Aerogels at all protonation levels withstood over 0.8compressive strain, showing near complete recovery once the load wasreleased. Specifically, the unloading curves returned to zero at ε=0 atall three 0.4, 0.6 and 0.8 incremental strains, showing complete shaperecovery even at high strains. In addition, all aerogels display inverselinear relationships of their wet-compressive strength at all strainlevels with increasing protonation. At 0.8 strain the compressive stressof the 11, 46, and 100% COOH aerogels were 19.3, 15.7, and 3.3 kPa,respectively (FIG. 26A-26D). The lower wet compressive strength of themore protonated CNF aerogels may be attributed to lower densities andthinner cell wall geometry.

Our previously reported 25.3 kPa maximum compressive stress at ε=0.8 forthe aerogel with similar density (8.1 mg cm′) and protonation level (11%COOH), but from more oxidized (1.29 mmol g⁻¹) CNF. The slightly highercompressive stress of 25.3 kPa than 19.3 kPa of lower oxidation (1.17mmol g⁻¹) at comparable density and protonation suggest the level ofoxidation influences such property.

Aerogels fully saturated and immersed in water (pH 5.7) disintegratedwithin 24-48 h under static state and can be quickened to 24-32 h underagitation. The wet-compressive strength of aerogels fully immersed inwater under cyclic loading-unloading at ε=0.6 strain over time (0, 2, 4,6 and 32 h) show similar hysteresis, but lowered maximum stress (FIG.27A-27C). The compressive strength reduced by over a quarter within thefirst 2 hours, i.e. from approximately 5 kPa to 3.5 kPa. At 32 h themaximum wet compressive stress at ε=0.6 strain was halved, and theloading curve showed an aberration to which was attributed thefracturing of the aerogel at its core (FIG. 27A, inset). Hence, whilethe 11% protonated aerogels are capable of cyclic absorption-squeezingwith fully recoverable pore structure to maintain high absorptivecapacity, the maximum stress at ε=0.6 strain deteriorate over time.

Most impressively, is that aerogel wet stability was enhanced by eitherprotonation of the CNFs, or increased acidity of the immersing media.Remarkably, as well, was that superior wet stability of aerogels couldbe achieved when saturated and immersed in nonpolar and aliphaticdecane, which could open new opportunities for application in otherorganic liquids.

Characterization of Aerogels by FTIR, TGA and XRD.

FTIR spectra of all three CNF aerogels showed typical cellulosecharacteristics peaks at 3394 cm⁻¹ (O—H stretching), 2911 cm⁻¹ (C—Hstretching), 1477 cm⁻¹ (H—C—H and O—C—H in-plane deformation), 1371 cm⁻¹(C—H deformation vibration), 1317 cm⁻¹ (H—C—H wagging vibration), 1201cm⁻¹ (C—O—H in-plane deformation), 1163 cm⁻¹ (C—O—C asymmetric bridgestretching of the β-glycosidic linkage), 1057 cm⁻¹ (C—O—C pyranose ringskeletal vibration), and 897 cm⁻¹ (C₁—O—C₄ deformation of theβ-glycosidic linkage) (FIG. 28A). The major spectral differences amongthe three CNF aerogels resided at 1725 and 1612 cm⁻¹, corresponding tothe respective carbonyl stretching in COOH and COO⁻Na⁺. A sharp peak at1612 cm⁻¹ for the 11% COOH spectrum was indicative of substantial sodiumcarboxylate content to overlap with the O—H deformation peakcorresponding to absorbed water, generally appearing around 1632 cm⁻¹.As surface COOH content was increased from 11% to 46%, a small peak wasobserved at 1710 corresponding to the COOH carbonyl stretching. Thedisappearance of the 1612 cm⁻¹ peak in the 100% COOH spectrum furtherconfirmed the complete COO⁻Na⁺ to COOH conversion, consistent with theconductometric titration data, and with the O—H deformation ofhygroscopic water reappearing at 1631 cm⁻¹.

All aerogels exhibit characteristic cellulose Iβ crystal structure,showing peaks at around 14.7, 16.8 and 22.7° assigned to the 110, 110,and 200 crystallographic planes of the monoclinic lattice (FIG. 28B).The crystallinity index (CrI) of the starting cellulose is 84.4%; andthat of the 11% COOH is 75.4% and those of the 46% COOH and 100% COOHare 72.5 and 71.2%, very slight decrease with increasing protonation.The thermal stability of the aerogels were improved with increasingprotonation was examined using TGA (FIG. 28C-28D).

Aerogel Structure and Surfaces as Affected by Tubes.

The 11% COOH CNFs were fabricated into aerogels in hydrophilicborosilicate glass or hydrophobic polypropylene (PP) tubes, both 14 mmin diameter. The aerogel from glass showed uniform honeycomb poremorphology internally and on the surface whereas that from PP displayedheterogeneous and concentric internal pore structure but non-poroussurface (FIG. 29A-29F). While the aerogel formed in glass showed rapidwetting, transport and retention by water without any change in aerogeldimension that formed in PP displayed a concave indent upon depositionof a 10 μL droplet of water, attributed to the nonporous surface andlarger and more heterogeneous peripheral macropores toward smaller poresat the core of the aerogel (FIG. 30A-30F).

Although the foregoing invention has been described in some detail byway of illustration and example for purposes of clarity ofunderstanding, one of skill in the art will appreciate that certainchanges and modifications may be practiced within the scope of theappended claims. In addition, each reference provided herein isincorporated by reference in its entirety to the same extent as if eachreference was individually incorporated by reference. Where a conflictexists between the instant application and a reference provided herein,the instant application shall dominate.

What is claimed is:
 1. A method for preparing an aerogel or a foam, themethod comprising: (a) forming a reaction mixture comprising a cellulosenanofibril gel, a first solvent, and one or more crosslinking agentsunder conditions sufficient to crosslink the gel, wherein the one ormore crosslinking agents comprise one or more coated polymers, whereinthe one or more coated polymers comprise cellulose acetate; and (b)contacting the crosslinked gel with a second solvent under conditionssufficient to dry the crosslinked gel, thereby forming an aerogel orfoam.
 2. The method of claim 1, wherein the reaction mixture furthercomprises an acid and the conditions further comprise conditionssufficient to protonate the gel.
 3. The method of claim 2, wherein theacid comprises hydrochloric acid.
 4. The method of claim 1, wherein thefirst solvent comprises acetone, ethanol, dimethyl sulfoxide,dimethylformamide, toluene, chloroform, or a combination thereof.
 5. Themethod of claim 1, wherein the crosslinked gel is washed with the firstsolvent following the crosslinking step.
 6. The method of claim 1,wherein the second solvent comprises tert-butanol, hexane, or acombination thereof.
 7. The method of claim 1, wherein drying thecrosslinked gel comprises freeze-drying at a temperature of about −50°C. at a pressure of about 0.05 mbar.
 8. The method of claim 1, whereindrying the crosslinked gel comprises air drying at a temperature of atleast about 20° C.
 9. An aerogel or foam prepared by the method of claim1.